This invention relates to the dimming of high intensity discharge lamps in particular to low wattage ceramic metal halide lamps. Traditionally, the rare earth spectra of these lamps are composed of thulium, praseodymium, neodymium, lutetium, gadolinium, terbium, dysprosium, holmium, erbium emitters. In addition, sodium and thallium iodide and mercury are added to the fill chemistry. These lamps are designed to have a specific color temperature (CCT), a high color rendering index (CRI) and a high efficacy (LPW) at rated power. However, at reduced power levels as low as 50%, the photometric performance deteriorates substantially.
There is a need for metal halide lamps with good dimming properties. Discharge lamps with rare earth chemistry are very popular because of their excellent color quality and high luminous output. Such lamps perform very well at rated power, however at reduced power levels, their photometric performance is rather poor. In the case of dimming as low as 50%, it is very desirable that the metal halide lamps produce white light with minimum color change and efficacy loss. To achieve effective dimming, the CCT, Duv and CRI should remain unchanged. There should only be little loss of efficacy.
Typically, the light of commercial metal halide lamps that contain thallium iodide (TlI) for high efficacy turns from a white at rated power to a greenish color under dimming conditions. In addition, the efficacy of these lamps decreases significantly. The metal halide TlI is highly desired because of its high vapor pressure and its strong green radiation at 535 nm. However under dimming conditions, the partial pressure of thallium tends to dominate, in turn, causing a shift of the light color to a greenish hue.
Therefore, the objective of this invention is to provide a metal halide lamp that at rated power gives excellent CCT, CRI, Duv and efficacy. A further objective is to provide a lamp that when dimmed to as low as 50% power maintains its color temperature and white hue. Still a further objective of the present invention is to provide a metal halide lamp that when dimmed retains substantially good CCT, CRI, Duv and efficacy, as close to rated power performance as possible. Yet another objective of the present invention is to provide a new chemistry for ceramic metal halide lamps that are electrically retrofitable in existing fixtures.
Metal halide lamps comprising rare earth iodides have high luminous output and excellent color properties. Their spectrum consists of multiple lines of atomic and molecular radiation. The composition of the fill chemistry of a particular type is optimized at rated power without taking into account the performance at reduced power levels. When these metal halide lamps are dimmed for energy saving purposes, their photometric performance degrades significantly. Especially, lamps containing thallium iodide exhibit a strong greenish hue under dimming condition. For 150 W lamps in particular, the color temperature increases from 4339 K at 150 W to about 5285 K at 75 W. The Duv increases more than 17 points and the CRI drops from 95 at 150 W to 69 at 50% power. The efficacy drops from 80 lpw at rated power to 61 lpw at 50% power.
U.S. patent application Ser. No. 09/627,842 filed Jul. 28, 2000, by Zhu et al., having the same assignee, describes a thallium free metal halide lamp with magnesium halide (MgI2) filling for improved dimming properties. The invention relates to ceramic metal halide lamps with metal halide materials such as NaI, DyI3, HoI3 and TmI3. The substitution of TlI with MgI2 greatly improves the color properties at lower than rated power levels.
The rare earth material cerium iodide (CeI3) has been used in U.S. Pat. No. 5,973,453 from Van Vliet et al., particularly in relation with NaI. The metal halide lamps of this patent are obtained with a high luminescent efficacy. However, these lamps are not suitable for dimming and have rather poor color properties at rated and reduced power. CeI3 was also used in the U.S. Pat. No. 3,786,297 from Zollweg et al. to produce lamps with high efficacy.